JP3102005B2 - Positive active material for lithium secondary battery and method for producing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electrolyte lithium secondary battery having high energy density using lithium as a negative electrode active material, and more particularly to an improvement of the positive electrode active material thereof.

_2. Description of the Related Art As a lithium battery, a primary battery using MnO ₂ for a positive electrode has already been put to practical use. In the case of a lithium battery, the presence of moisture adversely affects battery performance.
O ₂ was heat-treated at a temperature of 250 ° C. to 400 ° C. to remove attached water and bound water, and used as a positive electrode of a lithium battery. As the crystal structure of MnO ₂ , as disclosed in JP-B-49-25571, γ heat-treated at a temperature of 250 ° C. to 350 ° C.
-Β-form, or β-form which was heat treated at a temperature of 350 ° C to 430 ° C as disclosed in US Patent No. 4,133,856. However, in subsequent studies, MnO ₂ heat-treated at 400 ° C. in air is also referred to as γ-β type MnO _2, and it is said that bound water cannot be completely removed.

Further, it is said that if the bound water is completely removed, the γ-β type cannot be maintained, resulting in β-type MnO ₂ having extremely low activity as a battery active material. Further, it is known that even when the γ-β type is maintained, the capacitance characteristics deteriorate as the heat treatment temperature increases. This is partly because the surface of the active material partially changed to β type, but it is mainly caused by a decrease in surface activity such as partial reduction of the active material surface. Taking these facts into account, 350
Γ-β type MnO ₂ which slightly leaves bound water heat-treated at a temperature of about 400 ° C. to 400 ° C. is used in a lithium battery. However, when MnO ₂ having this crystal structure is used as a lithium secondary battery, the initial capacity is high and the energy density is high, but the capacity decreases with the cycle due to the collapse of the crystal structure due to charge and discharge. Further, it is said that residual bound water flows out due to the collapse of the crystal structure, which adversely affects battery performance, particularly cycle characteristics and storage performance. In addition, this active material has a high capacity at normal temperature, for example, in an environment of 20 ° C.,
The discharge capacity at low temperature is extremely low, for example,-
When the temperature reaches 20 ° C., the capacity is reduced to about 30% of the capacity at 20 ° C. This is unique to the secondary battery, and the Li / MnO ₂ primary battery does not have such a remarkable decrease in capacity at such a low temperature.

Therefore, in the current lithium secondary battery using γ-β type MnO ₂ as an active material, cycle reversibility, storage performance and low-temperature characteristics are insufficient, and some improvement seems to be necessary.
Therefore, from the viewpoint of cycle reversibility, manganese oxides including improvement of MnO ₂ have been actively developed, and some proposals have been made. In particular, attempts to improve the crystal structure by doping MnO ₂ with Li to improve the cycle reversibility have been made most actively.

For example, heat treatment of Li-doped MnO ₂ as disclosed in JP-A-62-108455 and JP-A-62-108457 is used. As to the active material of the lithium secondary battery, MnO ₂ doped with Li showed improvement in cycle reversibility as compared with conventional MnO ₂ . This is considered to be the effect that the crystal structure was reinforced by Li doping. In particular, among those reported so far, MnO ₂ mixed with about 30 mol% of LiNO ₃ and heat-treated at 400 ° C. are the most excellent in terms of cycle reversibility and active material utilization. In particular, the storage performance of this active material is not sufficient, but γ-β
It was improved compared to type MnO ₂ .

However, like the conventional MnO ₂ , this active material also has a very low discharge capacity at low temperatures, and its capacity drops to about 30% of the capacity at 20 ° C. at −20 ° C. There was a disadvantage that it would. That is, the conventional active material is insufficient from the viewpoint of reliability.

An object of the present invention is to provide a highly reliable lithium secondary battery having excellent energy density and cycle reversibility. A main object of the present invention is to provide an active material having improved capacity, cycle reversibility, storage characteristics, and low-temperature characteristics by improving MnO ₂ .

Means for Solving the Problems The present invention provides Mn: P = 1.00: 0.02 to 1.00: 0.10, and Mn: L
i = manganese having an atomic ratio of 1.00: 0.10 to 1.00: 0.40 (M
An oxide comprising n), phosphorus (R) and lithium (Li) is used as an active material. In the production method, the raw material containing Mn is electrolytic manganese dioxide (EMD), the raw material containing P is Li ₃ PO ₄ or P ₂ O _5, and the raw material containing Li is Li
₃ and PO ₄ or LiNO _3, these aforementioned Mn: P: mixed such that the atomic ratio of Li, in the air 350 ° C. or higher, which is then burned in a temperature range of 480 ° C. or less. In addition, EMD and P
It is preferable to use water as a medium when mixing the raw material containing Li and the raw material containing Li, and dissolve any of Li ₃ PO ₄ , P ₂ O ₅ or LiNO ₃ in water in advance, and then calcinate at the predetermined temperature. . The above problem can be solved by using the active material and the production method of the present invention described above.

Operation As a result of the X-ray diffraction analysis of the oxide comprising Mn, P, and Li of the present invention, the existence of a new park that cannot be partially analyzed from the diffraction pattern was confirmed. This is the conventional γ-β
It was not found in the active material doped with Li using the type MnO ₂ or LiNO ₃ . The presence of this new peak suggests that some different crystallization phase has occurred, but the details are not clear.

It is generally said that when Li enters the MnO ₂ crystal, the electron conductivity of the material itself decreases. It is said that this occurs even when performed thermochemically (doping using LiNO ₃ ) or electrochemically (discharge). That is, the conventional active material is used in a state of low electron conductivity. However, comparing the specific resistance after discharge of the conventional active material after discharging MnO ₂ or the active material doped with Li using LiON ₃ and the specific resistance after discharge of the active material of the present invention, It was found that the resistance of the active material of the invention was low. That is, when P is present as a kind of impurity in MnO ₂ as in the active material of the present invention, a bond is formed between tetravalent Mn and pentavalent P, and the semiconductor is formed by controlling the valence. It can be assumed that:

That is, the improvement in the electronic conductivity of the active material itself is considered as one of the causes of the improvement in the low-temperature characteristics. Also,
Conventionally, heat treatment for removing the bound water of MnO ₂ and heat treatment for doping Li have been performed. However, heat treatment generally reduces the surface area of MnO ₂ remarkably (almost the surface area of the original MnO ₂ is reduced). 20% to 40%). However, for the active material of the present invention, found that the results were compared with the surface area after firing the surface area before firing MnO ₂ (measured by the BET method), it is maintained more than 80% of the original surface area Was. This surface area maintaining effect is thought to be due to the P component, but details thereof are not clear. Probably, either of these or these act in combination to contribute to the improvement of low-temperature properties.

The active material of the present invention also has excellent storage characteristics, which is presumably due to the following reasons.
Although the P component in this active material seems to be distributed uniformly, it is expected that the active material has a partial form of P ₂ O _{5 on} the surface of the active material. In other words, since P ₂ O ₅ with extremely high water adsorption is on the active material surface, the bound water that has flowed out is trapped here and does not move to the negative electrode. It is unlikely to happen. As described above, although some assumptions can be made about various performance improvement mechanisms, the cycle reversibility, storage performance, and low-temperature characteristics are simultaneously improved by using the active material and the production method of the present invention. The facts are very interesting.

Examples Examples of the present invention will be described below.

(Example 1) The active material of the present invention was prepared as follows. First, the raw material Li ₃ PO ₄ or P ₂ O ₅ containing a predetermined amount of P, and the raw material Li ₃ PO ₄ or LiNO ₃ containing Li are dissolved in water in advance, and a predetermined amount of EMD powder is added thereto. After sufficiently stirring and mixing and partially evaporating water to form a muddy mass, the mixture is fired at a predetermined temperature for 4 to 5 hours using an electric furnace. Also,
Regardless of which mixing method was used, such as a method in which various materials were previously mixed as powders and then water was added and kneaded, the same material could be prepared in the subsequent firing. However, it was found that if the mixture of the powders was directly baked without using water, the reaction was difficult to be performed uniformly, and the performance variation was large. Furthermore, when P ₂ O ₅ was used as the material of the P component without using water, the P content after preparation was reduced as compared with the charged amount of the P component. Conventionally, it is said that P ₂ O ₅ has several types of crystal forms, and one type sublimates when the temperature exceeds 350 ° C. Probably, it contained such a form of P ₂ O ₅ . However, when any type of P ₂ O ₅ is dissolved in water, it becomes orthophosphoric acid,
It is said that when it is heated again, it changes to a stable mold that is extremely difficult to sublime. That is, the production method involving water at the time of mixing as in the present invention can be said to have an important meaning in this case.

Next, as a result of examining the relationship between the charged mixture ratio of the Mn component, the P component, and the Li component and the ratio of Mn, P, and Li in the active material by chemical analysis of the active material, according to the preparation method of the present invention, It was found that within the firing temperature range of the present invention, neither Mn nor P nor Li was lost, and the active material was contained at the ratio of the charged amounts.

(Example 2) EMD was used as MnO ₂ , and M was used in an atomic ratio according to the above-mentioned preparation method.
n: P: Li 1.00: 0.05: 0.3 was mixed with various materials such that the active material of the fired present invention at 400 ° C., conventional MnO ₂ active material was heat-treated at 400 ° C. The EMD, a LiNO ₃ EMD Mn: Li mixed with water and calcined at 400 ° C. Mn: Li is 1.00: 0.3 Li-doped MnO ₂ active material, and P ₂ O ₅ are mixed with EMD using water 4
A Pn-containing MnO ₂ active material having Mn: P of 1.00: 0.05 fired at 00 ° C was prepared. First, several button-type batteries as shown in FIG. 2 were assembled for these four active materials, and their characteristics were compared.

In FIG. 2, the positive electrode 1 is composed of an active material in which carbon powder of a conductive agent (5% by weight based on the active material) and polytetrafluoroethylene resin powder of a binder (7% by weight based on the active material) are mixed. This is press-formed on a titanium net 2 fixed by spot welding inside the positive electrode case. The amount of active material was 100 mg in each case. A gasket made of polypropylene together with a separator 5 made of polypropylene, a negative electrode 5 of metallic lithium pressed on a sealing plate 4 and an electrolytic solution 6 (1 mol / LiAsF ₆ dissolved in a mixed solvent of propylene carbonate and ethylene carbonate). 7 sealed through diameter
The battery is 20mm and 1.6mm high. This battery was prototyped for the purpose of comparing the characteristics of the positive electrode. The capacity of the negative electrode was filled about four times the capacity of the positive electrode, so that the charge-discharge characteristics were not affected by lack of the negative electrode. ing.
Charge / discharge test: 1.0mA constant current charge / discharge
The test was performed by setting 3.8 V and the discharge end voltage to 2.0 V. FIG. 1 shows the capacity-cycle characteristics of a battery using the above four types of active materials. In FIG. 1, curve 8 represents the conventional Mn.
The characteristic of the O ₂ active material, curve 9 is the characteristic of the Li-doped active material, curve 10 is the Pn-containing MnO ₂ active material, and curve 11 is the characteristic of the active material of the present invention. A battery using a conventional MnO ₂ active material has a large capacity at the beginning of a cycle, but has a large capacity decrease accompanying the cycle. The battery using the Li-doped MnO ₂ active material has a very large capacity and excellent cycle reversibility. The battery using the Pn-containing MnO ₂ active material has a small capacity at the beginning of the cycle, but after 10 cycles, exceeds the capacity characteristics of the conventional MnO ₂ active material, and has excellent cycle reversibility thereafter. Furthermore the active material of the present invention is the behavior of capacitance in the same way the initial cycle and MnO ₂ active material containing P is gradually increased is characteristic, and MnO ₂ active material doped with Li at 10th cycle The capacities are almost equal, and the subsequent cycle reversibility is also excellent. From the above results, the Li-doped MnO ₂ active material and the active material of the present invention are excellent in capacity, and the cycle reversibility includes Li-doped MnO ₂ active material, the active material of the present invention, and P Mn
It can be said that the O ₂ active material is excellent. Next, with respect to the above four types of batteries, the batteries were taken out in the state of charge at the 30th cycle, stored for one month in an environment of 60 ° C., and changes in internal resistance before and after storage were measured. Each battery had an internal resistance of 5 to 10 Ω before storage. Internal resistance after storage is
Battery with MnO ₂ active material 40-50Ω, Li doped
In the battery using the MnO ₂ active material, the value was 20 to 30Ω, and in the battery using the Pn-containing MnO ₂ active material and the active material of the present invention, the value was 10 to 15Ω. Therefore, a charge / discharge test was performed again on the four types of batteries subjected to the storage.

FIG. 3 shows a comparison of the capacity-cycle characteristics when the above-mentioned storage is included in the middle (30th cycle). The battery using the conventional MnO ₂ active material (curve 12) was doped with Li.
It can be seen that the capacity of the battery using the MnO ₂ active material (curve 13) is greatly reduced after storage. However, in the battery using the MnO ₂ active material containing P (curve 14) and the battery using the active material of the present invention (curve 15), the reduction in capacity is extremely small, and it can be said that the storage characteristics are excellent.

Next, charge / discharge tests were performed on the above four types of batteries in a low-temperature environment of room temperature (20 ° C.) and −20 ° C., and their discharge characteristics were compared. FIG. 4 shows the discharge voltage characteristics at the 30th cycle of each battery. In the battery using the conventional MnO ₂ active material, the battery was compared with the battery at room temperature (broken curve 16).
It can be seen that the voltage at −20 ° C. (curve 17) is lower and the capacity is about 30% of room temperature. Also. Even in the battery using the Li-doped MnO ₂ active material, the characteristic at −20 ° C. (curve 19) is extremely poor compared to the characteristic at room temperature (dashed curve 18). However, in the battery using the MnO ₂ active material containing P, the characteristic at −20 ° C. (curve 21) is lower than the characteristic at room temperature (dashed curve 20), but the capacity is lower than that at room temperature.
More than 70% was maintained. Furthermore, the battery using the active material of the present invention also has a -20 ° C.
(Curve 23) maintained more than 70% in capacity.

As described above, the active material of the present invention can be said to be extremely excellent in any of the capacity characteristics, cycle reversibility, storage performance, and low-temperature characteristics.

Next, using chemically synthesized manganese dioxide (CMD) as a raw material, the active material of the present invention prepared under the same composition and under the same conditions as above was also examined. As a result, the button-type battery using the same weight of the active material showed almost the same excellent performance as that of the EMD. However, as a result of measuring the bulk density,
If the positive electrode is nearly 0% bulky and has the same shape and the same size (generally, the size is restricted for a practical battery), the merit of the CMD is small. Therefore, in order to realize a high energy density, the raw material MnO ₂ is preferably EMD.

(Example 3) As described above, since it was found that the active material of the present invention exhibited excellent performance, the firing temperature according to the production method was examined next. A battery under the same conditions as in Example 2 for each active material prepared by mixing various materials so that Mn: P: Li = 1.00: 0.05: 0.3, and preparing various firing temperatures varied from 300 ° C to 500 ° C. And a charge / discharge test was performed. The charge / discharge test was performed at a constant current charge / discharge of 1.0 mA, with the charge end voltage set to 3.8 V and the discharge end voltage set to 2.0 V. FIG. 5 shows the capacity-cycle characteristics of typical active materials. In the case where the firing temperature is 300 ° C. to 340 ° C., the initial capacity is large as shown in the curve 24 (340 ° C.) in FIG.
490 ° C to 500 ° C exhibited excellent cycle reversibility as shown by curve 25 (490 ° C), but had a small capacity.
In the case where the firing temperature is 350 ° C to 480 ° C, curve 26 (350
° C), curve 27 (400 ° C), curve 28 (450 ° C) and curve 29
(480 ° C), both the capacity and the cycle reversibility were excellent. Therefore, from the capacity-cycle characteristics, the firing temperature of the active material of the present invention is 350 ° C. to 480 ° C.
Is preferred. In addition, all of the active materials of the present invention prepared in this temperature range exhibited excellent storage performance and low-temperature characteristics of the battery as in the above-described examples.

Example 4 In this example, the atomic ratio of Mn to P and Li in the active material (Mn:
P: Li) was studied. The method for preparing the active material was as described in Example 1, and the firing temperature in this example was 400 ° C. Maximum P ratio 0.15 and maximum Li ratio for Mn ratio 1.00
Various active materials were prepared at 0.5. Next, a battery under the same conditions as in the above example was formed for each, and a charge / discharge test was performed. The charge / discharge test was performed at a constant current charge / discharge of 1.0 mA, with the charge end voltage set to 3.8 V and the discharge end voltage set to 2.0 V. FIGS. 6, 7, and 8 show the capacity-cycle characteristics of typical active materials.

First, FIG. 6 shows the results of the study on the case where the P ratio was less than 0.02 and the Li ratio was variously changed with respect to the Mn ratio of 1.00. The active material having a Li ratio of less than 0.10 is curve 30 (Mn: P: Li = 1.00:
0.01: 0.09), it is close to the characteristics of the conventional γ-β type MnO ₂ (broken line in the figure), and although the initial capacity is large, there is a difficulty in cycle reversibility. The active material having a Li ratio of 0.10 to 0.40 was excellent in both capacity and cycle reversibility, as shown by curve 31 (Mn: P: Li = 1.00: 0.01: 0.30). However,
The capacity starts to decrease around the point where the Li ratio exceeds 0.40, and the curve
As seen in 32 (Mn: P: Li = 1.00: 0.01: 0.45), the cycle reversibility was excellent and the capacity was low.

On the other hand, when the P ratio was less than 0.02, there was a problem in storage performance and low-temperature characteristics regardless of the Li ratio.

Next, with respect to the Mn ratio of 1.00, the P ratio is set to 0.02 to 0.10,
FIG. 7 shows the results of the study on various changes in the Li ratio. In this P ratio range, any of the active materials having a Li ratio of less than 0.10 gradually increases in capacity at the beginning of the cycle as shown by curve 33 (Mn: P: Li = 1.00: 0.05: 0.09). The capacity is almost the same as the capacity characteristic of the Pn-containing MnO ₂ active material used in Example 2 (see curve 10 in FIG. 1). In the case of the active material having a Li ratio of 0.10 to 0.40 in this P ratio range, the capacity gradually increases at the beginning of the cycle as shown by a curve 34 (Mn: P: Li = 1.00: 0.05: 0.30). However, the capacity was as large as that of the Li-doped MnO ₂ active material used in Example 2 (see curve 9 in FIG. 1), and the cycle reversibility was excellent. However, even in this P ratio range, the capacity began to decrease around the point where the Li ratio exceeded 0.40, and the cycle reversibility was excellent, as shown by the curve 35 (Mn: P: Li = 1.00: 0.05: 0.45). But the capacity is low. On the other hand, the active material having a P ratio in the range of 0.02 to 0.1 exhibited excellent storage performance and low-temperature characteristics regardless of the Li ratio.

Next, with respect to the Mn ratio of 1.00, the P ratio was set to 0.11 to 0.15,
FIG. 8 shows the results of the study on various changes in the Li ratio. In this P ratio range, any active material having a Li ratio of less than 0.10 has a small initial capacity as shown by a curve 36 (Mn: P: Li = 1.00: 0.11: 0.09), and the capacity gradually increases with the cycle. However, 50 cycles or more must elapse before the capacity characteristic (dashed curve 39) of the conventional γ-β type MnO ₂ is reached, which is not preferable. Also,
In the range of the P ratio, in the case of the active material having the Li ratio of 0.10 to 0.40, the initial capacity is small as shown in the curve 37 (Mn: P: Li = 1.00: 0.11: 0.30), and the capacity increases with the cycle. Although it gradually increases, 50 cycles or more must elapse before reaching the capacity characteristic (broken line curve 40) of the Li-doped MnO ₂ active material used in Example 2, which is also not preferable. Further, in this P ratio range, those having a Li ratio exceeding 0.40 have extremely small capacities, as shown by curve 38 (Mn: P: Li = 1.00: 0.11: 0.45), and the capacities gradually increase with the cycles. However, the capacity characteristics of the conventional γ-β type MnO ₂ were not reached even after more than 100 cycles. However, the active material in this P ratio range was excellent only in storage characteristics in any Li ratio.

From the above results, the composition range of the active material of the present invention that satisfies any of the capacity characteristics, cycle reversibility, storage performance, and low-temperature characteristics is as follows: Mn: P is 1.00: 0.02 to 1.00 to 0.1.
0 and Mn: Li = 1.00: 0.10 to 1.00: 0.40. The firing temperature during the preparation of the active material within the above composition range was 350 in Example 3 in each case.
C. to 480 C. were preferred.

According to the present invention, a lithium secondary battery having high energy density, excellent cycle reversibility, and excellent storage performance and low-temperature characteristics can be provided.

[Brief description of the drawings]

1, 3, 5, 6, 7, and 8 are comparison diagrams of capacity-cycle characteristics, and FIG. 2 is a longitudinal section of a battery used in an embodiment of the present invention. FIG. 4 is a discharge voltage characteristic diagram. 1 ... Positive electrode, 2 ... Titanium net, 3 ... Separator,
4 ... sealing plate, 5 ... lithium negative electrode, 6 ... electrolyte solution, 7
……gasket.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Nobuo Eda 1006 Kadoma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. In-company (72) Inventor Hiromi Okuno 1006 Kadoma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. Reference JP-A-64-67869 (JP, A) JP-A-62-108455 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/36-4/62 H01M 4 / 02-4/04 H01M 10/40

Claims (3)

(57) [Claims] (1) Mn: P in an atomic ratio of 1.00: 0.02 to 1.00: 0.10,
And a lithium secondary battery characterized by being an oxide obtained by mixing and sintering electrolytic manganese dioxide, Li ₃ PO ₄ and LiNO ₃ so that Mn: Li = 1.00: 0.10 to 1.00: 0.40. For positive electrode active material. 2. The raw material containing Mn is electrolytic manganese dioxide (EM).
D), the raw material containing P is Li ₃ PO ₄ , the raw material containing Li is LiNO ₃ , and these are converted from Mn: P: Li to 1.00: 0.02 to
A method for producing a positive electrode active material for a lithium secondary battery, comprising mixing at an atomic ratio of 0.10: 0.10 to 0.40 and firing in air in a temperature range of 350 ° C. or more and 480 ° C. or less. 3. A method according to claim 1, wherein water is used as a medium when mixing the EMD, the raw material containing P, and the raw material containing Li, and Li ₃ PO ₄ or Li
The method for producing a positive electrode active material for a lithium secondary battery according to claim 2, wherein any one of NO ₃ is dissolved in water in advance and then calcined at the predetermined temperature.